The present invention relates to liquid extraction processes and, more particularly, to an improvement in the liquid-liquid extraction process for the recovery and partitioning of actinide values from acidic nuclear reactor waste aqueous solutions using bidentate organophosphorus compounds.
Solvent extraction processes which employ bidentate organophosphorus extractants are presently being actively developed at U.S. Energy Research and Development Administration Facilities in Hanford, Washington, and at Idaho Falls, Idaho, for the removal and recovery of the actinide elements, particularly americium and plutonium, from nitric acid waste solutions generated at these sites. In general, the bidentate organophosphorus compounds are efficient extractants of actinide values which are present in trivalent, tetravalent, and hexavalent oxidation states in the acidic nuclear waste aqueous solutions. With the bidentate extractant, essentially all actinide values, e.g. Am(III), Cm(III), Pu(IV), Np(IV) and U(VI) are extracted into the organic phase and, thereafter, the actinides are selectively stripped into trivalent, tetravalent and hexavalent fractions by contact with dilute aqueous acidic or alkaline solutions. One key feature of these processes is that they involve the use of dilute HNO.sub.3 to selectively strip trivalent (Am, Cm) actinides from coextracted tetravalent and hexavalent actinides. U.S. Pat. No. 3,993,728, issued Nov. 23, 1976, describes the application of bidentate organophosphorus extractants in actinide removal schemes from various acid nuclear fuel processing solutions such as those generated in the Purex process.
Currently preferred bidentate extractants are dihexyl-N,N-diethylcarbamylmethylenephosphonate, hereinafter referred to as DHDECMP, and dibutyl-N,N-diethylcarbamylmethylenephosphonate, hereinafter referred to as DBDECMP. These extractants and other neutral bidentate organophosphorus compounds are generally available as crude products which contain various impurities. One of these impurities is an unidentified acidic compound which has a very great affinity for actinide elements at low (e.g. less than 0.25 M HNO.sub.3) aqueous phase acidities. Thus, the DHDECMP or DBDECMP extractants must be purified of this acidic component to permit the use of low acid solutions to partition the trivalent actinides, i.e. trivalent amercium, from the other co-extracted actinides.
At present, the only known methods for purifying the commercially available DHDECMP or DBDECMP from the offending unknown acidic contaminant are by vacuum distillation or by contacting the organic extractants with a 6 M HCL solution at 60.degree. C. for 24 to 48 hours and washing the resulting organic phase at 25.degree. C. with equal volume portions of 1 M NaOH, 1 M HNO.sub.3 and HO. Both of these purification procedures have a number of disadvantages. Specialized vacuum distillation equipment is expensive and vacuum distilled DHDECMP or DBDECMP would cost more than twice as much as technical grade material. Specialized, corrosion resistant equipment is needed to perform HCL hydrolysis of the bidentate extractants, but more importantly, the HCl also attacks the DHDECMP as well as the acid impurity. Thus the acid hydrolysis procedure requires precise and careful control to produce a satisfactory extractant. Therefore, there is a need for a simple, reliable and inexpensive purification procedure which can be carried out in conventional, stainless steel equipment.